Instantaneous scarfing by means of a pilot puddle

ABSTRACT

A process for spot scarfing the surface of a metal workpiece comprising: 
     (a) impinging a stream of pilot oxygen gas upon a portion of the workpiece which is at least at its oxygen ignition temperature, said stream of pilot oxygen gas being narrower than the width of a desired scarfing cut, 
     (b) causing relative motion between said workpiece and said stream of pilot oxygen gas, so as to continuously produce a pilot puddle of molten metal along a chosen path on the surface of the workpiece, 
     (c) contacting said pilot puddle with a high intensity stream of oxygen gas so as to spread the puddle to a preselected width when said puddle reaches an area to be spot scarfed on the surface of said workpiece, and 
     (d) scarfing said area by impinging a stream of scarfing oxygen gas on the spread puddle, said stream of scarfing oxygen gas being wider than the pilot oxygen stream.

This is a continuation of Ser. No. 24,597 filed Mar. 28, 1979, now abandoned.

BACKGROUND

This invention relates to the scarfing of metal workpieces wherein defects on the surface of a workpiece are removed by a stream of scarfing oxygen gas. More specifically, this invention relates to a method and apparatus for spot scarfing one or more areas of the workpiece surface while the scarfing apparatus and workpiece are in motion relative to each other at normal scarfing speed.

Scarfing cuts are conventionally started by first preheating a band of metal on the surface of a workpiece to its oxygen-ignition temperature. The width of the band is normally equal to the width of the desired scarfing cut. A stream of scarfing oxygen is then impinged upon the preheated band and relative movement is provided between the scarfing oxygen stream and the workpiece, thereby producing the desired scarfing cut. During the preheating step of such a conventional process, which may take as long as 20 seconds or more, there can be no relative motion between the workpiece and the scarfing apparatus, since relative motion would prevent the band from becoming preheated to the required temperature. This period during which there can be no relative motion makes such conventional starts impractical for spot scarfing individual defects, because the frequent need to stop the relative motion for preheating would cause the spot scarfing operation to take too long if several defects had to be scarfed. Furthermore, if several scarfing units are arranged side-by-side to make a gang pass over the surface of the workpiece, stopping relative motion to make a start with one unit, while another unit is making a scarfing cut, would cause the scarfing stream of the unit making the cut to gouge an unacceptably deep groove in the workpiece during the period of no relative motion.

U.S. Pat. Nos. 3,991,985 and 3,996,503, as well as 4,038,108 describe methods and apparatus for instantaneously starting a scarfing reaction without stopping for preheating. While these inventions represent significant advances in the art, they both have the disadvantage of requiring relatively expensive and complicated equipment. The apparatus of U.S. Pat. Nos. 3,991,985 and 3,996,503 requires a wire feeding mechanism, and that of U.S. Pat. No. 4,038,108 requires a laser. Moreover, both of the inventions require starting a new reaction each time a scarfing cut is to be made, thereby necessitating frequent use of the wire feeder or laser.

OBJECTS

Accordingly, it is an object of this invention to provide a method and apparatus for spot scarfing the surface of a workpiece without interrupting the normal relative speed of travel between the workpiece and the apparatus, and without use of a wire feeder or laser.

It is another object of this invention to provide a method and apparatus for spot scarfing the surface of a workpiece without starting a thermochemical reaction each time a spot scarfing cut is to be made.

SUMMARY OF THE INVENTION

These and other objects which will become apparent to those skilled in the art are achieved by the present invention, one aspect of which comprises:

a process for spot scarfing the surface of a metal workpiece comprising:

(a) impinging a stream of pilot oxygen gas upon a portion of the workpiece which is at least at its oxygen ignition temperature, said stream of pilot oxygen gas being narrower than the width of a desired scarfing cut,

(b) causing relative motion between said workpiece and said stream of pilot oxygen gas, so as to continuously produce a pilot puddle of molten metal along a chosen path on the surface of the workpiece,

(c) contacting said pilot puddle with a high intensity stream of oxygen gas so as to spread the puddle to a preselected width when said puddle reaches an area to be spot scarfed on the surface of said workpiece, and

(d) scarfing said area by impinging a stream of scarfing oxygen gas on the spread puddle, said stream of scarfing oxygen gas being wider than the pilot oxygen stream.

The portion of the workpiece upon which the stream of pilot oxygen gas is impinged is preferably at its melting temperature.

A second aspect of the invention comprises:

spot scarfing apparatus comprising in combination:

(a) means for raising a portion of the surface of a metal workpiece to at least its oxygen ignition temperature,

(b) means for directing a stream of pilot oxygen gas against said heated portion to form and to continuously sustain a pilot puddle, said means being capable of discharging a stream of oxygen narrower than the width of the spot scarfing cut desired,

(c) means for spreading said puddle to a preselected width, said means being capable of discharging a high intensity stream of oxygen gas directed at said pilot puddle, and

(d) means for producing a spot scarfing cut wider than said pilot puddle, said means being capable of discharging a stream of scarfing oxygen gas directed at said spread puddle.

This invention is predicated on the discovery that a very narrow pilot puddle can be sustained on the surface of a workpiece moving at normal scarfing speed with a low-intensity stream of oxygen gas, and that the size of the pilot puddle can be expanded suddenly to the full width of a desired scarfing cut by directing a high intensity stream of oxygen at the pilot puddle. After the pilot puddle has been expanded to the desired width, the area of the workpiece sought to be spot scarfed is scarfed with a stream of scarfing oxygen gas. In sustaining the pilot puddle, i.e. in continuously producing a pilot puddle of molten metal along a chosen path on the surface of the workpiece, a very narrow and shallow cut will be made on the surface of the workpiece. However, this "pilot cut" is so small that very little metal is wasted, and the pilot cut itself will not become an undesirable surface defect.

The term "high intensity stream of oxygen", as used throughout the present specification and claims, is intended to mean a stream of oxygen having an intensity higher than that of the stream of scarfing oxygen. Contacting the pilot puddle with such a stream will abruptly broaden the width of the puddle to a preselected width; preferably, that of the desired scarfing cut.

The term "stream of pilot oxygen" as used in the specification and claims is intended to mean a stream of oxygen gas whose width is substantially narrower than the width of a desired scarfing cut. The stream of pilot oxygen will preferably have an intensity lower than that of the scarfing oxygen stream.

The term "stream of scarfing oxygen" as used herein, is intended to mean a stream of oxygen gas directed at an acute angle to the workpiece for the purpose of thermochemically removing defects from said surface.

THE DRAWINGS

FIG. 1 is a side view illustrating scarfing apparatus in accordance with the present invention capable of sustaining a pilot puddle and of suddenly expanding same to a preselected width.

FIG. 2 is a partial front view of the apparatus of FIG. 1, taken along line 2--2.

FIG. 3 illustrates the manner in which spot scarfing cuts can be made on the surface of a workpiece utilizing the apparatus of FIGS. 1 and 2.

FIGS. 4-6 illustrate alternative embodiments of the pilot oxygen nozzle used in the present invention.

FIG. 7 illustrates spot scarfing cuts made in accordance with the present invention when using several scarfing units mounted side-by-side in a "gang pass" arrangement.

FIG. 8 illustrates "gang pass" scarfing cuts made by a scarfing machine in accordance with the invention that has been programmed to minimize oxygen and fuel gas consumption.

FIG. 9 illustrates a sub-assembly useful for converting conventional scarfing apparatus to enable it to carry out the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate a preferred embodiment of the invention. Typically, a scarfing unit U is comprised of a head 1, a shoe 2, an upper preheat block 3, and a lower preheat block 4. A slot-like scarfing nozzle 9, from which a sheet-like stream of scarfing oxygen is discharged, is formed by the upper surface 7 of lower preheat block 4 and the lower surface 8 of upper preheat block 3. Shoe 2 rides on the surface of workpiece W, maintaining constant distances between the work surface and the scarfing unit U. Oxygen and fuel gas are supplied to the unit U by pipes 5 and 6, respectively, and thereafter to the various appropriate nozzles by conventional passages and flow control means (not completely shown) well known to those skilled in the art. Rows of ports 10 and 16 (see FIG. 2) in upper preheat block 3 discharge fuel gas and oxygen, respectively. A row of ports 11 in lower preheat block 4 discharge fuel gas.

Conduit C_(P) supplies oxygen to pilot oxygen nozzle 14 (see FIG. 2), which discharges a narrow stream of pilot oxygen gas to sustain the pilot puddle. Pilot oxygen nozzle 14 is located within scarfing oxygen slot nozzle 9. Conduit C_(s) supplies oxygen to the parts of scarfing oxygen slot nozzle 9 located outside of pilot oxygen nozzle 14, and valves V₃ and V₄ control the flow of pilot oxygen and scarfing oxygen respectively. There may be more than one oxygen supply conduit conveying oxygen to the scarfing unit, and valves V₃ and V₄ may be located external to the scarfing unit.

High intensity blowpipe 12 is mounted to discharge a high intensity stream of oxygen to spread out the pilot puddle sustained by nozzle 14 to the width desired for a scarfing cut.

It is preferable that upper preheat block 3 be designed to produce a post-mixed preheating flame in accordance with Fuhrhop, U.S. Pat. No. 4,115,154, the entire disclosure of which is incorporated herein by reference.

The functioning of the apparatus is best understood with reference to FIG. 3, which illustrates typical spot scarfing cuts made on the surface of a workpiece by the apparatus described in FIGS. 1 and 2.

In FIG. 3, scarfing cuts 23 and 25 would be made as follows. The end 20 of workpiece W is moved into register with scarfing unit U, (at the opposite end of the workpiece from that shown in the drawing) and relative motion between the workpiece and scarfing unit is stopped for the first and only time during the spot scarfing process. Fuel and oxygen gas are discharged from ports 10 and 16 and ignited, forming a preheat flame that impinges upon the small area 21 on the end 20 of the workpiece. The flame preheats portion 21 (P in FIG. 1) to at least its oxygen ignition temperature, preferably to its melting temperature. A narrow stream of pilot oxygen, directed at the heated portion 21, is then caused to be discharged from pilot nozzle 14 by partially opening valve V₃. The pilot oxygen reacts exothermically with the heated portion 21 of the workpiece forming a molten puddle. Relative motion at normal scarfing speed is commenced between the scarfing unit U and the workpiece W. The scarfing unit passes over the workpiece in the general direction of arrow A. The pilot oxygen stream continuously produces a pilot puddle of molten metal along chosen path 22 on the surface of workpiece W. Flames formed by igniting gases discharged from nozzles 16, 10 and 11 can be used to help sustain the pilot puddle, although such flames are not necessary.

When the pilot puddle reaches area 23, which contains a defect to be scarfed, the following sequence of events takes place without interruption of the relative motion. Blowpipe 12 is caused to discharge a high intensity stream of oxygen which impinges upon pilot puddle 22. The pilot puddle is abruptly spread to width X, a preselected width equal to that of the desired scarfing cut. A stream of scarfing oxygen is simultaneously discharged from slot nozzle 9 onto the spread puddle by opening valve V₄, and fully opening valve V₃ so that the oxygen flow from nozzle 14 is increased from the pilot oxygen intensity to scarfing intensity. As relative motion continues in the direction of arrow A, area 23 is scarfed out. Preheat flames discharged from ports 11 help sustain the scarfing reaction.

After scarfing cut 23 has been made, the scarfing oxygen of nozzle 9 is turned off by closing valve V₄ and partially closing valve V₃ so that the flow of oxygen from nozzle 14 is reduced to pilot oxygen stream intensity. Hot or molten metal remains at edge 23A of cut 23. Pilot oxygen from nozzle 14 impinges upon the hot or molten edge 23A and a pilot puddle is sustained along chosen path 24, in the same manner as for path 22. The path along which the pilot puddle is continuously produced does not have to be a straight line, but may follow any chosen path on the work surface. When area 25, containing another defect, is reached, the pilot puddle is spread suddenly by a high intensity stream of oxygen discharged from blowpipe 12, and area 25 is scarfed by again fully opening valves V₃ and V₄. After this second scarfing cut has been made, the pilot puddle 26 may be sustained by oxygen from nozzle 14 until the scarfing unit has passed over the workpiece.

The apparatus illustrated in FIGS. 1 and 2 may be mounted to individually spot scarf defects located anywhere on the surface of a workpiece. An example of such mounting appears in FIG. 7 of U.S. Pat. No. 3,991,985. If an individual fin-free cut is desired, the scarfing method disclosed in U.S. Pat. No. 4,040,871, may be used.

For best results, the intensity and width of the stream of pilot oxygen gas discharged from nozzle 14 should be just sufficient to sustain the pilot puddle. In this manner, very little metal will be removed from the path followed by the pilot puddle. With the apparatus illustrated in FIGS. 1 and 2, it is preferable that the discharge opening of pilot oxygen nozzle 14 be a square measuring about 6 mm per side.

If the initial temperature of the workpiece is below 760° C., the pilot oxygen stream intensity should be within the range of 40 to 70 SCMH/cm² of nozzle discharge area; preferably, it should be about 65 SCMH/cm². If the workpiece temperature is above 760° C., the pilot oxygen stream intensity should be from 30 to 45 SCMH/cm² of nozzle discharge area; preferably, about 40 SCMH/cm². At these intensities, a 6 mm square pilot nozzle arranged as shown in FIGS. 1 and 2 will produce a pilot puddle about 10 to 20 mm wide. The greater the distance between the workpiece and the mouth of the pilot oxygen nozzle, the greater the pilot oxygen stream intensity required. Different arrangements for the pilot oxygen nozzle are discussed later with regard to FIGS. 4, 5, 6 and 9.

When initial workpiece temperature is below 760° C., the high intensity stream of oxygen should preferably have an intensity of from 100 to 200 SCMH/cm² of nozzle discharge area, with about 115 SCMH/cm² being most preferred. It has been found in this instance, i.e., when initial workpiece temperature is less than 760° C., that an intensity of 115 SCMH/cm² discharged from a blowpipe of 20 mm diameter will spread a pilot puddle to a width of about 100 mm. For many applications, a spot scarfing cut of this width may be sufficient; however, if a wider cut is desired, a 35 mm diameter blowpipe discharging at 115 SCMH/cm² will spread a pilot puddle to a width of about 200 mm or; a 45 mm diameter blowpipe will spread it to a width of about 300 mm. The intensity of the high intensity stream of oxygen should preferably be from 70 to 150 SCMH/cm² of nozzle discharge area when the workpiece temperature is above 760° C. with 95 SCMH/cm² being most preferred. In this case, i.e., when initial workpiece temperature is above 760° C., said intensity of 95 SCMH/cm² discharged from a 20 mm diameter blowpipe will spread a pilot puddle to a width of about 100 mm, and if discharged from a 35 mm or 45 mm diameter blowpipe it will spread a pilot puddle to a width of about 200 mm or 300 mm, respectively.

Scarfing oxygen stream intensity should preferably be from 40 to 100 SCMH/cm² of nozzle discharge area when scarfing workpieces having an initial temperature below 760° C., with about 85 SCMH/cm² being most preferred. When initial workpiece temperature is above 760° C., scarfing oxygen stream intensity should preferably be from 45 to 70 SCMH/cm², with 55 SCMH/cm² being most preferred.

The spreading of the pilot puddle takes place suddenly, i.e. almost instantaneously. If the rate of relative motion between the workpiece and the scarfing apparatus is about 6 meters/minute and the initial temperature of the steel is below 760° C., the amount of travel while the puddle is being spread (dimension T in FIG. 3) will be less than half the width X of the scarfing cut. If the rate of relative motion is faster, i.e. about 20 meters/minute, dimension T will be about equal to the width of the scarfing cut, which still constitutes an extremely rapid spreading of the pilot puddle on cold steel. When scarfing steel having an initial temperature above 760° C., dimension T will be shorter.

All the preheat ports 10 and 16 may be used to perform the preheating, or the scarfing unit may be provided with control means (not shown) so that only the preheat ports located near nozzle 14 are used for preheating. A pre-mixed flame, that is, a flame formed by igniting oxygen and fuel gas that have been mixed within the scarfing unit may be used. However, it is preferable for purposes of greater safety to use a post-mixed flame, wherein oxygen and fuel gas are mixed outside the unit. Acceptable post-mixed preheating methods and apparatus are disclosed by Allmang and Lytle in U.S. Pat. Nos. 3,231,431 and 3,752,460. As previously mentioned, the method of Fuhrhop's U.S. Pat. No. 4,115,154, is preferred for producing a post-mixed flame. However, any method of heating a portion of the surface of the workpiece to its oxygen ignition temperature or melting temperature can be used, be it an electrical arc, or any other energy concentrating system.

FIGS. 4 to 6 illustrate alternate preferred arrangements for supplying pilot oxygen gas to the surface of the workpiece. In FIG. 4 pilot oxygen nozzle 14A is located within upper preheat block 3. The apparatus illustrated in FIG. 5 has the pilot oxygen nozzle 14B located within lower preheat block 4. FIG. 6 is a side view illustrating still another arrangement for providing pilot oxygen. In FIG. 6, the pilot oxygen nozzle is a pipe 14C, located external to the scarfing unit above upper preheat block 3. Any nozzle arrangement capable of supplying pilot oxygen to a suitably located heated portion on the surface of the workpiece may be used.

Two or more scarfing units constructed in accordance with the present invention may be mounted parallel to each other to make a "gang pass", i.e. simultaneous parallel passes by several scarfing units, over the workpiece. Nozzles so arranged may be mounted on a gantry as illustrated in FIG. 9 of U.S. Pat. No. 3,991,985. If fin-free cuts are desired, the gang-pass-mounted nozzles may be of the type disclosed in U.S. Pat. No. 4,013,486.

FIG. 7 illustrates scarfing cuts made by apparatus comprising 3 units (31, 32 and 33) mounted side-by-side for a gang-pass over workpiece W. Scarfing units 31, 32 and 33 are constructed like those illustrated in FIGS. 1 and 2 and function in the same manner. FIG. 7 illustrates the position of the apparatus after the scarfing operation has been completed. The scarfing takes place as follows: edge 34 of workpiece W is positioned in register with the three scarfing units. Portions 35 and 36 of edge 34 are heated to at least oxygen ignition temperature by units 31 and 32 respectively, as described previously. Since an area 37 containing a defect is located close to edge 34, flames discharged from ports 10 and 16 of unit 33 heat band 38A on the surface at edge 34 to its oxygen ignition temperature over width 38, equal to the full width of the desired scarfing cut. All 3 units accomplish heating of their respective portions of the surface of the workpiece bordering edge 34 simultaneously. Then the pilot oxygen streams of scarfing units 31 and 32 and the scarfing oxygen stream of unit 33 are turned on, and the units are moved along the workpiece in the direction of arrow A at scarfing speed. The pilot oxygen streams from units 31 and 32 sustain pilot puddles along paths 39 and 40, while unit 33 makes scarfing cut 37 with a stream of scarfing oxygen. When area 41 is reached, unit 31 is turned on and suddenly spreads its pilot puddle with its high intensity blowpipe to the width of the desired scarfing cut and scarfs area 41. After area 41 has been scarfed, the scarfing oxygen stream of unit 31 is turned off, but the pilot oxygen stream is left on, sustaining a pilot puddle along path 42. After area 37 has been scarfed, the scarfing oxygen of unit 33 is turned off, leaving the pilot oxygen on to sustain the pilot puddle along path 43. When area 44 is reached, unit 33 suddenly spreads its pilot puddle to the desired width and area 44 is scarfed. Since there is no defect in the surface passed over by unit 32, that unit performs no scarfing during the entire pass.

It is important to note that, after relative motion starts between the scarfing units and the workpiece, the motion continues uninterrupted at the desired scarfing speed throughout the entire pass. If the motion had to be interrupted, for example to start cut 41, the scarfing oxygen of unit 33 would gouge a deep groove in the workpiece in area 37.

Relative motion between the workpiece and the scarfing apparatus may be along any chosen path, and may be provided by any desired means; either may move while the other is stationary, or both may move simultaneously. The means for producing said motion may be an integral part of the scarfing machinery, for example, as shown in FIGS. 7 and 9 of U.S. Pat. No. 3,991,985, mentioned previously. Alternately, or in combination with the above, relative motion means external to the scarfing machine may be employed, for example, a steel mill's roll table which moves a workpiece in relation to a scarfing apparatus.

FIG. 8 illustrates a workpiece scarfed by apparatus of the present invention that is programmed to conserve oxygen and fuel gas.

Instead of starting pilot puddles at edge 50, the scarfing apparatus may be positioned at the edge of the first area to be scarfed, area 56, a distance "d" from edge 50 and halted there. While the apparatus is halted, units 31 and 33 preheat portions 51 and 52 and each unit's pilot oxygen is turned on. Simultaneously, a conventional scarfing start is made by unit 32 by preheating area 53 and turning on the scarfing oxygen. Relative motion between the gang pass apparatus and the workpiece is then immediately commenced, sustaining pilot puddles along paths 54 and 55 and making a scarfing cut over area 56. After area 56 has been scarfed, the scarfing oxygen on unit 32 is turned off, leaving pilot oxygen on to sustain a pilot puddle along path 58. When area 57, containing a defect, is encountered, unit 31 spreads its pilot puddle and scarfs the defect. Similarly, unit 33 scarfs area 60 when that area is encountered. After area 57 has been scarfed, unit 31 is turned off completely, since that unit has no further defects to scarf. When area 59 is encountered it is scarfed by unit 32.

The programming method used to control the flows of gases from the various nozzles forms no part of this invention. Such programming could be accomplished manually by an operator who starts and stops gas flow from the appropriate nozzles at the appropriate times. Preferably, the sequences for starting and sustaining the pilot puddle, and for spreading the puddle and scarfing a defect will be carried out automatically by sequencing apparatus. Defect detection apparatus could be used to detect defects and send signals to sequencing apparatus which could cause the defects to be automatically scarfed out of the workpiece. Programming apparatus into which the pattern of defects on a workpiece is pre-recorded before actual scarfing occurs may also be used.

The present invention may be used in conjunction with the instantaneous starting methods disclosed in previously-mentioned U.S. Pat. Nos. 3,996,503 and 4,038,108. A first scarfing cut may be started by either the wire or laser method disclosed by said patents. After a cut has been made, a pilot puddle is sustained in the same way as that which was sustained along path 24 of FIG. 3, for example. Subsequent scarfing cuts are made by spreading the pilot puddle with a high intensity stream of oxygen and scarfing the desired area as described previously. This method has the advantage of not requiring a delay for preheating prior to the first scarfing cut, yet not requiring use of the wire feeder or laser to start subsequent cuts.

FIG. 9 illustrates a sub-assembly useful for enabling conventional scarfing apparatus to carry out the method of the invention. U₁ represents any conventional scarfing unit having means (not shown) for preheating a portion of the surface of the workpiece to its oxygen ignition temperature and a scarfing nozzle 9. Sub-assembly S is connected to oxygen supply pipe or source 130. Oxygen is conducted from pipe 130 to pilot oxygen nozzle 14d and high intensity blowpipe 12d. The sub-assembly is shown mounted on scarfing unit U₁, such that the oxygen streams discharged from nozzles 12d and 14d impinge upon the spot on the workpiece preheated by unit U₁. The apparatus functions as follows: unit U₁ preheats spot P on the surface of the workpiece to its oxygen ignition temperature. Valve V₂ has only a small flow opening that allows only a low-intensity stream of oxygen to flow from pilot oxygen nozzle 14d. A small pilot puddle is sustained along a chosen path. A high-intensity stream of oxygen is impinged upon the pilot puddle by opening valve V₁ spreading the puddle to a preselected width. Valve V₁ has a large flow opening that allows a high-intensity stream of oxygen to flow from nozzle 12d. The defective area is scarfed with a stream of scarfing oxygen discharged from nozzle 9.

Of course, nozzles 12d and 14d may have separate supply pipes, and valves V₁ and V₂ or other means for controlling the flow of oxygen may be located external to the subassembly. 

What is claimed is:
 1. A process for spot scarfing the surface of a metal workpiece comprising:(a) impinging a stream of pilot oxygen gas upon a portion of the workpiece which is at least at its oxygen ignition temperature, said stream of pilot oxygen gas being narrower than the width of a desired scarfing cut, (b) causing relative motion between said workpiece and said stream of pilot oxygen gas, so as to continuously produce a pilot puddle of molten metal along a chosen path on the surface of the workpiece, (c) contacting said pilot puddle with a high intensity stream of oxygen gas so as to spread the puddle to a preselected width when said puddle reaches an area to be spot scarfed on said workpiece, and (d) scarfing said area by impinging a stream of scarfing oxygen gas on the spread puddle, said stream of scarfing oxygen gas being wider than the pilot oxygen stream.
 2. The process of claim 1 wherein said portion of the workpiece is at its melting temperature.
 3. The process of claims 1 or 2 further comprising the additional steps of:(e) shutting off the stream of scarfing oxygen gas after the selected area has been scarfed, while continuing to cause relative motion between the workpiece and the stream of pilot oxygen, so as to continue to produce said pilot puddle of molten metal along a chosen path on the surface of the workpiece, and (f) repeating steps (c) and (d) when another area to be scarfed is encountered by the pilot puddle.
 4. The process of claims 1 or 2 wherein said pilot puddle is characterized by being from about 10 to about 20 mm wide, and wherein said stream of scarfing oxygen is from about 100 mm to about 300 mm wide.
 5. The process of claims 1 or 2 wherein the initial temperature of said workpiece is below 760° C., the intensity of the pilot oxygen stream is from 40 to 70 SCMH/cm² of nozzle discharge area, the intensity of the scarfing oxygen stream is from 70 to 100 SCMH/cm² of nozzle discharge are, and the intensity of the high intensity stream of oxygen is from 100 to 200 SCMH/cm² of nozzle discharge area.
 6. The process of claim 3 wherein the initial temperature of said workpiece is below 760° C., the intensity of the pilot oxygen stream is from 40 to 70 SCMH/cm² of nozzle discharge area, the intensity of the scarfing oxygen stream is from 70 to 100 SCMH/cm² of nozzle discharge area, and the intensity of the high intensity stream of oxygen is from 100 to 200 SCMH/cm² of nozzle discharge area.
 7. The process of claim 4 wherein the initial temperature of said workpiece is below 760° C., the intensity of the pilot oxygen stream is from 40 to 70 SCMH/cm² of nozzle discharge area, the intensity of the scarfing oxygen stream is from 70 to 100 SCMH/cm² of nozzle discharge area, and the intensity of the high intensity stream of oxygen is from 100 to 200 SCMH/cm² of nozzle discharge area.
 8. The process of claims 1 or 2 wherein the initial temperature of said workpiece is above 760° C., the intensity of the pilot oxygen stream is from 30 to 45 SCMH/cm² of nozzle discharge area, the intensity of the scarfing oxygen stream is from 45 to 70 SCMH/cm² of nozzle discharge area, and the intensity of the high intensity stream of oxygen is from 70 to 150 SCMH/cm² of nozzle discharge area.
 9. The process of claim 3 wherein the initial temperature of said workpiece is above 760° C., the intensity of the pilot oxygen stream is from 30 to 45 SCMH/cm² of nozzle discharge area, the intensity of the scarfing oxygen stream is from 45 to 70 SCMH/cm² of nozzle discharge area, and the intensity of the high intensity stream of oxygen is from 70 to 150 SCMH/cm² of nozzle discharge area.
 10. The process of claim 4 wherein the initial temperature of said workpiece is above 760° C., the intensity of the pilot oxygen stream is from 30 to 45 SCMH/cm² of nozzle discharge area, the intensity of the scarfing oxygen stream is from 45 to 70 SCMH/cm² of nozzle discharge area, and the intensity of the high intensity stream of oxygen is from 70 to 150 SCMH/cm² of nozzle discharge area.
 11. Spot scarfing apparatus comprising in combination:(a) means for raising a selected portion of the surface of a metal workpiece to at least its oxygen ignition temperature, (b) means for forming and continuously sustaining a pilot puddle by directing a stream of pilot oxygen gas against said heated portion on the workpiece surface, said means being capable of discharging a stream of oxygen narrower than the width of the spot scarfing cut desired, (c) means for spreading said puddle to a preselected width, said means being capable of discharging a high intensity stream of oxygen gas directed at said pilot puddle, and (d) means for producing a spot scarfing cut of preselected width, said means being capable of discharging a stream of scarfing oxygen gas directed at said spread puddle.
 12. The apparatus of claim 11 wherein said means for producing said spot scarfing cut comprises a slotted nozzle formed by the lower surface of an upper preheat block and the upper surface of a lower preheat block.
 13. The apparatus of claim 12 wherein said means for forming and sustaining a pilot puddle is located within said upper preheat block.
 14. The apparatus of claim 12 wherein said means for forming and sustaining a pilot puddle is located within said lower preheat block.
 15. The apparatus of claim 11 wherein said means for forming and sustaining a pilot puddle is located within said means for producing said spot scarfing cut, and comprises separate conduit means for conveying oxygen to each of said means.
 16. A sub-assembly, for use in combination with thermochemical scarfing apparatus, capable of producing a pilot puddle and of spreading it to a preselected width, comprising:(a) means for forming and continuously sustaining a pilot puddle by directing a stream of pilot oxygen gas against a selected portion of the surface of a workpiece heated to at least its oxygen ignition temperature, said means being capable of discharging a stream of oxygen narrower than the width of a spot scarfing cut desired, (b) means for spreading said puddle to a preselected width, said means being capable of discharging a high intensity stream of oxygen gas directed at said pilot puddle, and (c) means for supplying oxygen gas to means (a) and (b). 